A. Technical Field
The present invention relates generally to ambient light sensing and proximity detection, and more particularly, to a double-layered ambient light sensor and proximity detector photodiode array having improved ambient light sensing and infrared sensitivity.
B. Background of the Invention
The importance of ambient light sensing and proximity detection is well understood by one of skill in the art. Both technologies have particular applicability to the mobile electronics market in that effective ambient light sensors and proximity detection allow a mobile device to properly manage power consumption and extend battery life. Typically, an ambient light sensor and a proximity detector operate in different wavelength bands and differ structurally within an integrated sensor array.
An ambient light sensor determines the intensity of visible light within the environment surrounding the sensor. In particular, the ambient light sensor provides a response to the amount of visible light (typically a narrow band of wavelengths centered around 550 nm) being detected within an environment. These sensors are oftentimes used in mobile devices to detect the amount of light present in the environment in which the device is being used. Based on this detection, the brightness or intensity of the display on the mobile device may be adjusted to provide an optimal intensity to the user while also properly managing power consumed by the display. One skilled in the art will recognize that there are many other applications for an ambient light detector.
FIG. 1A shows a curve that is commonly known as the CIE photoptic curve. This curve represents the relative spectral response of the human eye. It can be seen that the human eye is not sensitive to wavelengths below 400 nm or above 700 nm. It is desirable that the ambient light sensor provides a spectral response that matches this curve as closely as possible.
FIG. 1B shows a curve that represents the spectral response of a typical silicon photodiode within the prior art. It can be seen that the silicon photodiode is responsive to wavelengths starting at 300 nm, and extending to above 1000 nm. The peak responsivity is in the region around 800 nm. Comparing FIGS. 1A and 1B, it is evident that an un-modified silicon photodiode cannot be used as an accurate ambient light sensor because it is sensitive to wavelengths of light outside the range of human vision. The mismatch between the two curves is most evident in the infrared region between 700 nm and 900 nm because while the human eye is not responsive to wavelengths beyond 700 nm, the silicon photodiode is very responsive in this region. It is well known that almost all natural and artificial light sources contain varying amounts of infrared radiation in the range of 700 nm to 1000 nm. In particular, light from incandescent light bulbs and also from the sun contains substantial quantities of infrared radiation. Although humans can feel this radiation in the form of heat, they cannot see it. Consequently, in order to use the common silicon photodiode as an accurate ambient light sensor, it is necessary to modify its responsivity to more closely match the human eye response.
The most common and economical method employed to modify the spectral response of a silicon photodiode is to apply color filters on the surface of the photodiode. FIG. 1C shows the spectral response of a typical red filter 102, a typical green filter 103, and the combined response of the green filter applied on top of the red filter 104. A further refinement to this practice is to divide the ambient light sensing photodiodes into two equal, but separate parts. The green filter is applied to one part, and the combination of the green plus red filter is applied to the other part. The respective signals from these two photodiodes can be scaled and then electronically subtracted from each other to produce the curve shown in FIG. 1D which also includes a copy of the CIE curve for reference. It can be seen that the response of the electronically processed signal, G−(G+R) 108 is a close approximation to the desired CIE curve 107.
An optical proximity detector determines the presence or absence of a reflective target within a certain range of the sensor. The intended target can be any object that reflects light. These proximity detectors operate by emitting light, either pulsed or continuous, and then sensing the light reflected from the target. Among other applications, proximity sensors are used within mobile phone devices to detect a handset being placed next to an ear or face. During this period in which a user is on the telephone, power to the device screen and/or other applications may be reduced in order to conserve battery power. One skilled in the art will recognize that there are many other applications for a proximity detector.
Proximity detectors typically employ infrared emitting light sources as the light emitter. There are several reasons for this choice, among them the abundant availability of high performance LED light sources at the infrared wavelengths near 850 nm and 940 nm, the high sensitivity of silicon photo detectors in this wavelength region, and the desire for the proximity detect function to be invisible, and therefore undetectable by the user.
The reflectivity of the target can be extremely variable because of the unknown characteristics of the target. For example, in order to detect the proximity of a mobile telephone handset to the user's face, the proximity detector must be able to operate correctly with reflections from dark hair, light hair, dark clothing such as hats or scarves, and also bare skin with or without facial hair. This wide variation of reflectivity demands a very sensitive detector.
FIG. 1E illustrates an array of photodiodes 100 that provides both ambient light sensing and proximity detection commonly found within the prior art. The array comprises a P-type substrate 105 on which N-well photodiodes are constructed. In order to shape the response of the photodiodes, as described previously, a color filter layer or layers is placed over each of the photodiodes. A green filter 120 is placed on top of each of the N-well photodiodes and a red filter 110 (in addition to the green filter) is placed only on particular photodiodes. Typically, the number of photodiodes that receive both layers is equal to the number that receives only the green layer. A first set of these photodiodes operates as green and red N-well photodiodes 130 and a second set of these photodiodes operates as green N-well photodiodes 140.
The array shown in FIG. 1E performs ambient light sensing and proximity detection. The ambient light sensing function produces a spectral curve matching the CIE curve of FIG. 1A. As described previously, one of the important characteristics of the ambient light sensing function is the ability to suppress the otherwise strong sensitivity to infrared light. This function is performed by executing the mathematical operation G−(G+R) as previously described. The proximity detect function, however, has a much different requirement. It must have high infrared sensitivity in order to sense the presence or absence of a light at the infrared wavelengths of 850 nm or 940 nm. These two functions place opposite burdens on the photo sensor with respect to infrared detection—one function requires suppression of infrared, while the other function requires enhancement of infrared reception. It is difficult to satisfy these two conflicting demands in one sensor, and in practice a compromise is typically employed.
FIG. 2 illustrates exemplary responses of the green pixel response and red pixel response in prior art systems. A first plot 210 showing the green only sensor response is provided. A second plot 220 showing the red only response, a third plot 230 showing the green+red response, and a fourth plot 240 showing the green−K*(green+red) response is also provided, where K is a constant of proportionality. The unfiltered response of the photodiode is also provided in plot 250.
FIG. 3 shows the plot 310, the green−K*(green+red) response (which is the same as plot of 240), along with plot 320, which is the previously discussed CIE curve. One skilled in the art will recognize that the plot 310 obtained from processing the filtered signals from the photodiode provides a reasonably accurate representation of the CIE curve 320.
The plot 310 was obtained by executing the mathematical algorithm green−K*(green+red). Close observation of the green plot 210 and the K*(green+red) plot 230 reveal that in the infrared region between 700 nm and 1000 nm, the two plots match very well. That is why the resulting subtraction of these two signals, as exemplified by plot 310, has a small resulting response in the infrared wavelengths of interest. It is easily recognized that the accuracy of this subtraction is very much dependent upon obtaining the correct gain constant K, and also upon accurately performing the subtraction. Small errors in either the value of K, or in the subtraction operation, will result in large errors in the infrared response of the ambient light sensor. This sensitivity to errors is to a large extent a result of the fact that the two signals that are subtracted have large infrared sensitivities. The subtraction of two large numbers to obtain a small resultant is very sensitive to errors.
The same array of photodiodes shown in FIG. 1E is also used for the proximity detect function in prior art systems. It is evident from observing FIG. 2 that the signal of plot 230, the green+red signal, may be used for the proximity detect function. This signal has a broad peak response in the desired wavelengths of 850 nm and 940 nm, and has very little response in the visible wavelengths. Low sensitivity in the visible wavelengths is advantageous for a proximity detector in order to reduce false triggers from ambient visible light. Unfortunately, the robust response in the infrared wavelengths that is advantageous to the proximity detector function is at the same time disadvantageous to the ambient light sensor.
What is needed is an integrated light sensor and proximity detector in which the ambient light sensor has enhanced sensitivity to visible wavelengths and reduced sensitivity to infrared wavelengths and the proximity of detector has enhanced sensitivity to infrared wavelengths and reduced sensitivity to visible wavelengths.